1. Field of the Invention
The invention relates to pressure swing adsorption gas separation operations. More particularly, it relates to an improved flow distribution system for pressure swing adsorption vessels.
2. Description of the Prior Art
Adsorption processes have been widely used for the separation and purification of gases. In recent years, pressure swing adsorption (PSA) systems have been developed for enhanced gas separation operations, particularly for the commercial production of oxygen and/or nitrogen from air. In the operation of PSA systems, an adsorption-desorption-repressurization processing sequence is employed, with the passage of air or other feed gas mixture at an upper adsorption pressure to an adsorption system for the selective adsorption of a more readily adsorbable component of air or other feed gas mixture by one or more adsorbent beds, and the passage through said bed(s) of a less readily adsorbable component. The bed(s) is then depressurized to a lower desorption pressure for the desorption of the more readily adsorbable component from the adsorbent bed(s), and the bed(s) is then repressurized to the upper adsorption pressure as cyclic operations are continued.
In order to fully utilize the adsorbent material employed, PSA systems require uniform flow of gas across the adsorbent bed(s) throughout the PSA processing cycle. In addition, large void volumes and pressure drops in the PSA vessel entrance and exit regions, which have adverse effects on the process performance of a PSA system, need to be mitigated in practical commercial operations. In this regard, those skilled in the art will appreciate that, in PSA systems, e.g. vacuum pressure swing adsorption (VPSA) systems, the adsorbent bed support and the flow distribution system are costly, and become more costly as the diameter of an adsorbent bed increases. The typical bed support system is also prone to adsorbent leakage if not assembled correctly. Repair of a leaking bed support system can be costly and time-consuming. Common bed designs and flow distribution systems that employ bed support plates and associated flow distribution systems, incorporating support ribs, tend to give rise to localized non-uniform gas flow, which results in an undesired penalty in gas separation performance.
In the modern PSA technology, adsorbent beds are shorter and will go through a faster cycle. They are designed smaller with little end space. The prior art distributors were not able to provide satisfactory flow distributions, with reasonable void volumes. In fact, they were not responsive to the demands of modern technology. The goal of the invention is to make the vertical PSA vessels more efficient in their separation task by achieving a better flow distribution. PSA vessels are used to capture and reduce the contaminants of chemical components such as carbon dioxide, carbon monoxide, nitrogen, oxygen, water, and hydrocarbons of the feed stream to ppm levels. This is carried out by adsorbing gases on different adsorbents such as alumina, carbon, zeolite, and molecular sieves. The adsorbent is contained in vessel and is alternated between adsorption and purge steps. A vertical vessel has generally the bed at the middle of the vessel, and has two distributors, one over the bed and one below the bed. Vertical beds can be made as tall as 25 ft with a typical diameter of 9 ft. Flow maldistribution in PSA vessels can cause early breakthrough and loss of efficiency. Flow maldistributions can also cause sieve movement and local bed fluidization.
U.S. Pat. No. 5,298,226 relates, in general, to apparatus for providing uniform fluid flow in vessels having packing materials or particulates and, in particular, to apparatus for providing uniform gas flow in pressure swing adsorbent vessels.
U.S. Pat. No. 5,538,544 relates to a non-uniform, graded ball adsorbent bed support which is employed in the head section of an adsorption vessel to achieve a uniform flow of gas to the adsorbent bed. The open areas of an inlet gas distributor are also varied to channel gas toward the edge portion of the vessel head to enhance the uniform flow of gas to the adsorbent bed.
U.S. Pat. No. 4,065,391 relates to a distributor that supplies either a liquid or a gas to a bed of granular media, such as a sand filter, is divided by inclined interior walls into primary and secondary horizontal conduits that extend parallel to each other. Gas metering orifices are positioned at an intermediate level in the inclined walls and liquid metering orifices are positioned beneath the gas metering orifices in the inclined walls. A liquid or a gas is supplied to the primary conduits, passes through the metering orifices in the inclined walls. A liquid or a gas is supplied to the primary conduits, passes through the metering orifices into the secondary conduits, and through dispersion orifices in the secondary conduits into the filter bed. At least some of the secondary conduits are each connected to at least two primary conduits so that liquid can flow from one primary conduit through a secondary conduit to another primary conduit to compensate for inequalities of flow along the primary conduits.
U.S. Pat. No. 4,938,422 relates to an inlet distributor to prevent bed surface disturbances at high inlet velocities and high particle loadings. The distributor uses a series of partitions to peel off portions of the downward gas flow and redirect them radially outward. Each outwardly directed gas flow component passes through a series of perforations to effect any necessary circumferential redistribution before entering the space above the particle bed. By subdividing the gas flow into a number of radially directed flow portions and circumferentially redistributing these flow portions, cross-currents and eddy currents on the catalyst bed surface are minimized or avoided so that disturbances at the bed surface are eliminated. This distributor is particularly effective in vessels having particles loaded to within a short distance of bed inlets and where elbows or other upstream flow devices introduce nonuniformities into the gas flow to a particle bed. The distributor has a simple arrangement and can be used without adding significant pressure drop to the system.
U.S. Pat. No. 5,544,423 relates to a gas distributor for an inlet gas plenum comprising a plurality of spatially separate gas deflectors, each deflector arranged and positioned to successively cut diagonally across a gas flow path in a gas flow direction from an inlet and of the distributor to a terminal end of the distributor. The deflectors configured to direct gas flow exiting the distributor in an essentially perpendicular direction to the direction of gas flow entering the distributor.
European Patent Application EPO 748 253 B1 relates to a mixing assembly for gaseous flows at different temperatures, in particular for heterogeneous exothermic synthesis reactors comprising a plurality of superimposed catalytic beds and in which at least one of the beds is provided with a gas permeable wall for outlet of a flow of hot gas, said assembly including:                a baffle extending below and parallel to said gas permeable wall with which it defines a hollow space for collection of said hot gas flow;        an annular opening for outlet of said hot gas flow from said hollow space defined between said baffle and a side wall supporting said at least one bed associated wit the reactor;        a perforated distributor for a cooling gas flow supported below said baffle at a predetermined distance from said annular opening.        
U.S. Pat. No. 5,716,427 relates to equipment, for example of the PSA type, which comprises gas circulation elements for passing the gas horizontally through an adsorbent (3), which comprise, on at least one vertical side of the adsorbent, a gas distribution volume comprising a first subvolume (5; 9) adjacent to the adsorbent, and a second subvolume (6; 10) separated from the first subvolume by a wall provided with passages (40) having cross-sections and/or a distribution which are selected so as to reduce the variations in local flow rate along the adsorbent. The equipment is particularly useful in separating gases from air.
U.S. Pat. No. 5,779,773 relates to a grille (26) for retaining an active material (42) in a receptacle (10) which has a convex bottom (14) delimiting an internal space within the receptacle. The grille (26) has the form of a vault and is disposed in the internal space in contact about a portion of its periphery with the internal wall of the convex bottom of the receptacle. The direction of convexity of the vault (26) and the bottom (14) are opposite to each other. Use in grilles for the support of an adsorbent material in an adsorber.
Therefore, there is a genuine need to find a better way to engender a uniform velocity across the entire cross-section of the process vessels having packing materials or particles. The enhancement of fluid distribution, i.e., improved uniformity, allows for the use of all the available reaction or separation surface area in the process vessels, thereby efficiently increasing the yield of the desired product.
It is an object of the invention to provide an improved adsorbent vessel head section capable of achieving a uniform flow of gas to an adsorbent bed therein.
It is another object of the invention to provide an improved PSA vessel capable of achieving uniform gas flow drop.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.